Underground loop antenna



May 11,1965 L. E. RAwLs UNDERGROUND LOOP ANTENNA .2 Sheets-Sheet 1 FiledDec. 22, 1960 FIG. |A

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INVENTOR LUCIEN E. RAwLS BY Wo 072e/ W0@ ATTORNEYS May 11, 1965 E. RAwLs3,183,510

` UNDERGROUND LOOP ANTENNA Filed Dec.y 22. 1960 2 Sheets-Sheet 2INVENTOR. LUCIEN E. RAWLS BY 7X0@ @fw/Wo@ .i ATTORNEYS United StatesPatent O 3,183,510 UNDERGROUND LGOP ANTENNA Lucien E. Rawls, Leesburg,Va., assigner, by mesne assignments, to Deco Electronics, Inc.,Washington, D.C., a corporation of Virginia Filed Dec. 22, 1960, Ser.No. 77,778 3 Claims. (Cl. 343-719) The present invention relates tohardened antennas and particularly to a buried antenna capable ofwithstanding nuclear blasts or theY like.

It is an object of the present invention to provide an antenna whichpresents a small target cross section and which has an optimum launchingefficiency relative to its cost.

Another object of the invention is to provide a buried antenna whichradiates a vertical polarized field above ground.

It is another object of the invention to provide an efficient buriedantenna having a subs-tantially omnidirectional radiation pattern.

It is another object of the invention to provide a buried antenna whoseefficiency is not impaired by subfsurface and surface water.

According to the invention, radio waves, preferably below 500 kc., aretransmitted from a buried loop antenna or loop antenna array. Radiowaves in the frequencies ranging downward from 500 kc. are transmittedmore and more efficiently by a buried loop antenna or loop antenna arraythan by other antenna types of cornparable dimensions in terms ofwavelengths. Each loop antenna is located in a vertical plane and has alength of the order of a quarter wavelength of the radiated waves inair. The height or width of the antenna is small relative to its length.The antenna radiates vertically polarized waves in a fgure-of-eightpattern having its axis along the length of the loop and nulls at rightangles to the loop. To achieve an omnidirectional pattern, a pair ofloops may be arranged at right angles or four loops may be arranged inthe form of a turnstile. The antenna is located in low loss material andprovided with means for shedding surface water and draining sub-surfacewater to maintain a high launching efficiency.

Other objects and advantages of the invention will become apparent andthe invention itself will be fully understood from the followingdescription and the drawing wherein:

FIG. l is a schematic sectional view showing a loop antenna according tothe invention;

FIG. 2 is a cross sectional view taken along the line 2--2 of FIG. l;

FIG. 3 is an end View of a wire forming the loop antenna;

FIG. 4 is a sectional view showing another embodiment of the invention;

FIG. 5 is a schematic view of a two element orthogonal loop antennaarray; and

FIG. 6 is a schematic view of a quadrangular array of four loopantennas.

Referring to the drawing, a two-wire coaxial cable 1t) connects loopantenna 12 to a transmitter. The radio `Waves transmitted may beanywhere in the range below 500 kc., but preferably below 100 kc. Forthe sake of definitencss, the present invention will be describedparticularly with reference to an antenna operating at a frequency near500 kc. Wires 13, 14 of cable 10 and connections 15 and 16 feed arms 17to 2t) of the loop. The loop has a length of approximately a quarterwavelength in air of the Waves to be transmitted. The length of aquarter Wavelength is a rough upper limit to the overall length. Thelength may be more with some resultant ICC distortion of the figure 8horizontal radiation pattern of a single loop. If less, the efficiencywill be decreased. For a frequency of 500 kc. a suitable length of theloop is approximately 150 meters. Loop 12 is located in a vertical planewith arms 17 and 20 immediately above arms 18 and 19, respectively, andspaced therefrom a distance of about 3 feet. Loop 12 is tuned by acapacitor 21 connected between conductors 13 and 14.

Loop 12 is formed of a conductor having a plastic core 24 surrounded bya cylindrical stranded conductor 25, such as a heavy duty Copperweldwire. Conductor 25 is embedded in a plastic cover 26 formed of materialsuch as polystyrene having a low effective dielectric constant, say lessthan 3. The plastic cover 26 should have a relatively larger diameter,for example, 4V inches, when the diameter of conductor 2S is about 2inches. The larger part of the latter dimension is that of plastic core24. The plastic cover 26, having a low dielectric constant and low lossand relatively large diameter, reduces losses which would otherwiseoccur in the material surrounding the loop wires.

The construction of the antenna will now be described particularly withreference to FIG. t2. The ground is graded so as to provide a run-offfor surface water. For

this purpose a suitable incline 27, 28 is provided on both sides of theposition of the loop antenna, and this grad ing, of course, extends theentire length of the antenna. Three trenches 3d, 31 and 32, are dug to asuitable depth, for example, 10 feet. The trenches can have anyconvenient shape; for example, they can be triangular or V-shaped,having sloping sides, or straight sides, as shown in FIG. 2. Thesetrenches run the entire length of the loop antenna. At the bottom of theouter trenches drainage means are provided, such as clay tile 33 and 34.The clay tile drains extend the entire length of the loop antenna andslope downwardly from the center of the loop toward the two endsthereof. The tile may be in the form of pipe having openings therealongfor the admission of water. After the clay :tile 33 and 34 are laid atthe bottom of trenches 31 and 32, these trenches are re-filled with asuitable material which will impede the movement of water toward thecenter trench. Such a material is stabilized soil which will remain dry.Trench 3i) is filled to a suitable depth of, say, two feet, with silicasand or vermiculite. The loop antenna is then placed in the trench andconnected to the twin coaxial line 1@ as shown in FlG. l. The ltrench 30is further filled with coarse silica sand or vermiculite to a heightslightly above the loop antenna. The remainder of trench 3f) is thenfilled with stabilized soil to a depth of about l foot within thesurface. A plastic Pliofilm 40 is -then laid over trenches 30-32 for theentire length of the antenna to form a Waterproof shed over thetrenches. To increase th? effectiveness of lm 40, it is preferablyextended a few feet beyond the outer trenches 31, 32. Film 4t) is thencovered with earth 41 to hold down and hide the film and to provide asuitable slope of the earth to facilitate run-off of surface water.

In the embodiment of the invention shown in FIG. 4 a triangular trench47 has a low loss fill 42 such as concrete or processed, dried earth.Upper loop wire 43 is buried at about one meter, or deeper, and lowerloop wire 44 at one meter or more below the upper wire. A weatherproofcover or Pliofilm 45 is placed over the trench, slightly below thesurface of the earth, to shed surface water. lf the fill 42 issufficiently resistant to water, the side trenches 31 and 32 of FIG. 2may be dispensed with. The length of the loop depends on the frequency,a length of about 250 to 300 feet being suitable at 1 megacycle and alength of about 500v feet at 500 kc. Generally, the loop length and thenumber of Crossovers of the loop 3 cable wire are such that the phaseshift along the loop is negligible, the length being uncritical.

In order to provide an omnidirectional radiation pattern a pair of loopantennas 46, 48 are placed across each other as shown in FIG. 5.Antennas 46 and 48 are each connected by cables, as shown in FIG. 1, toa transmitter which energizes these loop antennas in phase quadrature.Another method of obtaining a substantially omnidirectional radiationpattern is to use four loop antennas of the type disclosed in FIG. 1,but arranged as in FIG. 6. Loops 51 and 53, FIG. 6, are preferablyenergized in opposite phase, as are loops 52 and 54, and loops 51 and 53are energized in phase quadrature with loops 52 and 54. All fourantennas 51 to 54 are connected by suitable cables to the sametransmitter. It is to be understood that the antennas in FIGS. and 6 areof the same construction and are buried in the same manner as theantenna of FIGS. 1 and 2.

As is indicated in patent application Serial No. 77,5 S7, tiled onDecember 22, 1960, in behalf of George F. Leydorf and Lucien E. Rawls,launching losses are significantly reduced if power losses occassionedby skin effect and proximity efr'ect are minimized. Advantageously, suchlosses are minimized if the subject antenna, or antenna array, is buriedbelow the ground surface at a depth of the order of one skin depth;i.e., about one skin depth from the surface of the ground to thelongitudinal central axis of the loop. Further, proximity effect lossesare reduced if the upper and lower loop conductors are spaced-apart byabout the order of a skin depth. Skin effect and proximity effect arewell known phenomena, which are explained in great detail in thefollowing books: Electronics and Radio Engineering 4th edition, authoredby Frederick E. Terman and published by McGraw-Hill Company, at pages 2l'to 24 and pages 803 to 808; and, American Institute of PhysicsHandbook, published in 1957 by the McGraw-Hill Company, at pages 5-85 to5-95. Brieily, the phenomenon called skin effect is the tendency of analternating, or varying, current to redistribute over the conductingmediums cross section in such a pattern that the current density isgreater where it is encircled by fewer lines of magnetic llux; proximityeiect being the term given to the skin eiect phenomenon in connectionwith interacting conducting media such as looped conductors and coils. Aradiated electromagnetic eld near a conducting medium induces a currentflow in the conducting medium: The current density in the conductingmedium being greatest near the inducing iield; the current densitydecreasing exponentially in the conducting medium at distances more andmore remote from plate where the eld is being applied. At a remotedistance, or depth of penetration, from the place where the eld is beingapplied, the current density will be about 36.8% of the maximum currentdensity.

`sions have been given.

The remote distance at which this reduction in current density occurs iscalled one skin depth.

For the sake of deniteness, a particular embodiment of the invention hasbeen described and particular dimen- It will be understood, however,that many variations and modications of the arrangements hereindisclosed will be apparent to those skilled in the art. For example,other meanswhich might be used for maintaining a low conductivity andlow loss in the vicinity of the antenna include lling theloop-containing trench with oil or applying heat to the trench.Therefore, it is not desired to limit the invention except vas deiinedin the following claims.

What is claimed is:

l. A loop antenna for transmitting radio waves, comprising a single turnloop having an upper conductor and a lower conductor located in avertical plane, a transmission line connected to said loop for feedingthe same, said loop, having a length approximately equal to a quarterwavelength in air ofthe radio waves transmitted thereby, the distancebetween the upper and lower wires of theloop being a very` smallfraction of the length of the loop, said loop being buried in a trenchwithin the earth with the center of said loop at a depth below theearths surface of no more thanone skin depth, said trench being filledto above the loop with a material having a lower loss than thesurrounding earth, separate drainage trenches on each side of said loopantenna extending the entire length thereof, and a liquid impermeablecover located above said trench and extending the entire length of theloop for shielding said loop antenna from surface water.

2. A loop antenna according to claim 1, wherein said drainage trenchesinclude pipe means at the bottoms thereof covered by a stabilized soil,and wherein said impermeable cover also covers said drainage trenches.

3. A loop antenna according to claim 2, wherein said material is coarsesilica sand.

References Cited by the Examiner UNITED STATES PATENTS 1,220,005! 3/ 17Lyon et al 343-719 1,365,579 1/21 Appleby et al. 343-719 X 1,373,6124/21 Hanson 343-719 1,530,129 3/25 Loftin et al. 343-719 2,690,509 9/54Toth 343-855 2,910,695 10/59 Troost et al. 343-855 X 2,980,793 4/61Daniel 343-719 X FOREIGN PATENTS 149,532 8/20 GreatfBritain. 224,661 11/24 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

BENNETT G. MILLER, Examiner.

1. A LOOP ANTENNA FOR TRANSMITTING RADIO WAVES, COMPRISING A SINGLE TURNLOOP HAVING AN UPPER CONDUCTOR AND A LOWER CONDUCTOR LOCATED IN AVERTICAL PLANE, A TRANSMISSION LINE CONNECTED TO SAID LOOP FOR FEEDINGTHE SAME, SAID LOOP HAVING A LENGTH APPROXIMATELY EQUAL TO A QUARTERWAVELENGTH IN AIR OF THE RADIO WAVES TRANSMITTED THEREBY, THE DISTANCEBETWEEN THE UPPER AND LOWER WIRES OF THE LOOP BEING A VERY SMALLFRACTION OF THE LENGTH OF THE LOOP, SAID LOOP BEING BURIED IN A TRENCHWITHIN THE EARTH WITH THE CENTER OF SAID LOOP AT A DEPTH BELOW THEEARTH''S SURFACE OF NO MORE THAN ONE SKIN DEPTH, SAID TRENCH BEINGFILLED TO ABOVE THE LOOP WITH A MATERIAL HAVING A LOWER LOSS THAN THESURROUNDING EARTH, SEPARATE DRAINAGE TRENCHES ON EACH SIDE OF SAID LOOPANTENNA EXTENDING THE ENTIRE LENGTH THEREOF, AND A LIQUID IMPERMEABLECOVER LOCATED ABOVE SAID TRENCH AND EXTENDING THE ENTIRE LENGTH OF THELOOP FOR SHIELDING SAID LOOP ANTENNA FROM SURFACE WATER.